Most small, simple, mainstream mods do work regardless of OS and are simply placed into your interface folder within the WoW folder. Other large ones such as Cosmos [if that's even still around] have a mac version.

I use approximately 50 some mods when playing Warcraft (sick I know) and they all work fine. Even mods with .exe's work on a Mac you just can't use whatever functionality an EXE might give you. Most of the EXE's extract data that the Warcraft mod gathers during gameplay for you to upload to a website. So it doesn't affect your gameplay really.

There is no reason a Mod should not work on the Mac side. It's coded in LUA and isn't even compiled to be OS specific.

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